Refinement of motion vectors in hierarchical motion estimation

ABSTRACT

A method of motion estimation (ME) refinement. The method generally includes the steps of (A) generating an initial motion vector (MV) by conducting a first ME on an initial block in a picture, the initial block covering an initial area of the picture, (B) generating a current MV by conducting a second ME on a current block in the picture, (i) the current block covering a subset of the initial area and (ii) the second ME being seeded by the initial MV, (C) generating at least one additional MV by conducting at least one third ME on the current block, the at least one third ME being seeded respectively by at least one neighboring MV spatially adjacent to the current MV and (D) generating a refined MV of the current block by storing in a memory a best among the current MV and the additional MV.

FIELD OF THE INVENTION

The present invention relates to digital video motion estimationgenerally and, more particularly, to a method and/or apparatus forimplementing refinement of motion vectors in a hierarchical motionestimation.

BACKGROUND OF THE INVENTION

In hierarchical motion estimation (ME), video frames undergo a processof resolution reduction. Several layers are constructed, each containingthe same frame as the previous layer, but having both dimensions reducedby a certain scaling factor. The scaling factor is usually a factor of2, a small power of 2, or the power of another integer. The result is apyramid, where the lowest layer has the original frame and each layerabove has the same frame at increasingly reduced resolutions.

After pyramids have been created for both a target frame being motionestimated and a reference frame, a full search takes place in thehighest layer for target regions. Hence, a search window is defined inthe reduced reference frame and all candidate motion vectors areevaluated over the search window for each reduced block in the targetframe. The motion vectors are found by comparing all of the blocks inthe search window to the reduced blocks in the target frame using acomparison metric (i.e., a sum of absolute difference (SAD) or a sum ofsquared differences (SSD)).

The resulting motion vectors are then propagated to the next layer downby multiplying each of the motion vector coordinates with the scalingfactor. The scaled vectors become the center of new searches in the nextlayer. Because of the resolution increase at the next layer, each of thescaled motion vectors actually becomes the search center of severaltarget regions in the next layer, as dictated by the scaling factor andrelative region sizes between each layer. For example, at a scalingfactor of 2×2 and equal region sizes, a single motion vector for aparticular region in the higher layer will seed four regions in the nextlayer below. Once seeded, the motion vectors in the next layer aremotion refined against the reference frame. The process of propagatingthe motion vectors and refining the motion vectors is repeated untilresults for the bottom layer are reached, where the process ends.

A common problem that arises in hierarchical searches is that anerroneous match in the higher layer usually propagates to the lowerlayers, often resulting in an erroneous motion vector. Erroneous motionvectors are not rare cases since the lower resolution of the higherlayers often leads to ambiguity in the motion estimations. A number ofcandidate blocks will have similar metric values (i.e., SADS) and whilethe initial motion vector selected may be slightly better in the higherlayer, the initial motion vector may not be better in the bottom layer.

Another common problem arises in hierarchical searches when a movingobject boundary falls in the middle of the target region at the higherlayers. In such a case, the motion estimation can lock to either side ofthe object, thus producing a wrong motion vector predictor for the otherside. At the next layer down, the motion vector predictor will beapplied to the entire target region even through part of the targetregion contains the moving object and another part contains a stationarybackground. If the next layer search range is not large enough tocompensate for the situation, (and many motion vector field smoothingtechniques, such as rate-distortion optimization, can prevent the motionestimation from fixing the situation even if the search range issufficient) the same motion vector will propagate into both regions andnothing is available to fix the motion vector of the other side. Assuch, dragging artifacts are commonly produced in the video.

An existing solution to the hierarchical search problems is to perform aconventional search. However, conventional searches use very largesearch ranges to capture high motion and/or high temporal separationthat are compounded at higher resolutions (i.e., high definitionframes). Otherwise, the conventional searches suffer degradedcompression efficiency for large frames.

Another existing solution to the hierarchical search problems is topropagate more than a single motion vector predictor from each targetregion down to the next layer. However, the increase in motion vectorpredictors results in more searches in the next layer, thereby increasescomputational complexity.

SUMMARY OF THE INVENTION

The present invention concerns a method of motion estimation refinement.The method generally comprises the steps of (A) generating an initialmotion vector by conducting a first motion estimation on an initialblock in a picture, the initial block covering an initial area of thepicture, (B) generating a current motion vector by conducting a secondmotion estimation on a current block in the picture, (i) the currentblock covering a subset of the initial area and (ii) the second motionestimation being seeded by the initial motion vector, (C) generating atleast one additional motion vector by conducting at least one thirdmotion estimation on the current block, the at least one third motionestimation being seeded respectively by at least one neighboring motionvector spatially adjacent to the current motion vector and (D)generating a refined motion vector of the current block by storing in amemory a best among the current motion vector and the additional motionvector.

The objects, features and advantages of the present invention includeproviding a method and/or apparatus for implementing refinement ofmotion vectors in a hierarchical motion estimation that may (i) usepreviously refined current layer neighboring motion vectors to refine acurrent motion vector, (ii) incur a limited increase in computationalcomplexity as compared with conventional techniques, (iii) provide asmooth vector field leading to improved compression efficiency, (iv)provide accurate tracking of the true motion of objects and/or (v) beapplied to non-hierarchical sub-block motion estimation designs.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 is a block diagram of an apparatus in accordance with a preferredembodiment of the present invention;

FIG. 2 is a diagram of example layers of a hierarchy of a currentpicture;

FIG. 3 is a flow diagram of an example method of a hierarchical motionestimation;

FIG. 4 is a flow diagram of a first example implementation of a motionestimation refinement method;

FIG. 5 is a flow diagram of a second example implementation of a motionestimation refinement method;

FIG. 6 is a flow diagram of a third example implementation of a motionestimation refinement method;

FIG. 7 is a diagram of an example picture divided into blocks andsub-blocks; and

FIG. 8 is a flow diagram of an example implementation of a method of asub-block motion estimation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a block diagram of an apparatus 100 is shown inaccordance with a preferred embodiment of the present invention. Theapparatus (or system) 100 generally uses the resulting motion vectors ofexternal blocks near the corners of a current block in a current layer(or level) of a hierarchical search to perform small search-rangerefinement searches. A current motion vector of the current block may bereplaced with a refinement motion vector if the motion estimation scoreof the refinement motion vector is lower than the motion estimationscore of the current motion vector. The refined motion vectors may thenbe used as inputs to the next layer below, where the process isrepeated.

Motion estimation is an operation that generally identifies how an area(macroblock, block or sub-block) of a current picture (e.g., field,frame or image) has moved relative to a similar area in one or morereference pictures. The motion estimation operation may create a singlemotion vector per area for a predictive type of coding and two motionvectors per area for a bidirectional type of coding. The referencepictures may exist temporally before and/or temporally after the currentpicture.

The apparatus 100 may be implemented as part of a digital video encoder.The apparatus 100 generally comprises a circuit (or module) 102 and acircuit (or module) 104. A signal (e.g., CUR) may be received by thecircuit 102. Another signal (e.g., REF) may also be received by thecircuit 102. A signal (e.g., MV) may be transferred between the circuit102 and the circuit 104.

The circuit 102 may implement a motion estimator. The circuit 102 isgenerally operational to calculate multiple motion vectors for a currentpicture against one or more reference pictures. Pixels of the currentpicture may be received via the signal CUR. Reference pixels of thereference pictures may be received via the signal REF. The estimatedmotion vectors may be exchanged with the circuit 104 in the signal MV.

The circuit 104 generally implements a memory. The circuit 104 may beoperational to store the motion vectors estimated by the circuit 102.The circuit 104 generally makes the motion vectors available back to thecircuit 102 and to compression techniques performed by the apparatus100.

Referring to FIG. 2, a diagram of example layers of a hierarchy 110 of acurrent picture is shown. The example is based on a three-layerhierarchy. Other numbers of layers may be implemented in the hierarchyto meet the criteria of a particular application.

A bottom layer 112 (only partially shown) of the hierarchy 110 generallycomprises the current (or bottom) picture undergoing motion estimation.A middle picture in the middle layer 114 may be created by downsamplingthe bottom picture per a scale factor in each of a horizontal dimensionand a vertical dimension (e.g., a 2H×2V scale factor). A top picture maybe created in the top layer 116 by downsampling the middle picture perthe scale factor. Therefore, (i) the middle picture is both spatiallysmaller and at a lower resolution than the bottom picture and (ii) thetop picture is both spatially smaller and at a lower resolution than themiddle picture. The one or more reference pictures may be downsized inthe same manner as the current picture.

Using a 2H×2V scale factor as an example, a 1920×1080 bottom picture maybe reduced to a 960×540 middle picture and a 480×270 top picture. Assuch, instead of performing a full motion estimation search for each ofthe 32,400 8×8 blocks in the bottom picture, the apparatus 100 may startby searching the only 2,025 8×8 blocks in the top picture. Usingdifferent scale factors between the middle layer 114 and the top layer116, and/or using more layers in the hierarchy 110 may reduce the numberof initial motion estimation searches to be performed.

Referring to FIG. 3, a flow diagram of an example method 120 of ahierarchical motion estimation is shown. The method (or process) 120 maybe implemented by the apparatus 100. The method 120 generally comprisesa step (or block) 122, a step (or block) 124, a step (or block) 126, astep (or block) 128, a step (or block) 130, a step (or block) 132, astep (or block) 134 and a step (or block) 136.

The motion estimation may begin in the step 122 where the circuit 102downsamples the current picture received in the signal CUR and thereference picture received in the signal REF to create two pyramids. Inthe step 124, the circuit 102 may divide the top picture into multipleblocks (e.g., 8×8 pixel blocks). In the example shown in FIG. 2, the toppicture (e.g., layer 116) may be divided into 12 blocks. The circuit 102may then conduct a motion estimation for each of the blocks in the toplayer in the step 126. In FIG. 2, the motion vectors of the top picturecalculated by the motion estimations are labeled A-L.

After finding some or all of the motion vectors in the top layer, thecircuit 102 may move down a layer in the hierarchy (e.g., to the middlelayer 114) in the step 128. The top layer motion vectors (e.g., A-L) maybe copied to corresponding blocks in the middle layer 114 in the step130 by the circuit 102. For example, the motion vector G in the toplayer 116 may be copied into all of the motion vectors G1, G2, G3 and G4in the middle layer 114. Similar copying may be performed by the circuit102 for the remaining motion vectors A-L.

Motion estimation refinement searches may be conducted for the middlelayer motion vectors A1-L4 by the circuit 102 in the step 132. Eachmotion estimation refinement search is generally simpler andcomputationally less complex that a full motion estimation search duethe search center seed vectors. If more lower layers exist in thehierarchy 110 (e.g., the YES branch of step 134), the circuit 102 mayrepeat the steps 128-132 to copy the refined motion vectors from themiddle layer 114 to the lower layer 112 and then refine the lower layermotion vectors. Once the motion vectors of the lowest layer (e.g., thecurrent picture) have been refined (e.g., the NO branch of step 134),the circuit 102 may copy the final refined motion vectors to the circuit104 for storage in the step 136.

The above method 120 may be repeated for each layer of the hierarchy 110and for each picture of the video content. For example, after refiningthe middle layer 114 the circuit 102 may move down to the bottom layer112 in the step 128. The refined motion vector G3 may then be copieddown as a search center seed to the four corresponding blocks in thebottom picture (e.g., G3 a=G3, G3 b=G3, G3 c=G3 and G3 d=G3) per step130. The motion vectors in the bottom picture may be refined by thecircuit 102 in the step 132. Afterwards, the refined motion vectors ofthe bottom picture in the bottom layer 112 may be copied to the circuit104 in the step 136 and the process ended.

Referring to FIG. 4, a flow diagram of a first example implementation ofa motion estimation refinement method 132 a is shown. The method (orprocess) 132 a may be implemented by the apparatus 102 and used as thestep 132 of the method 120. The method 132 a generally comprises a step(or block) 140 and a step (or block) 142.

By way of example, a refinement of the motion vectors at the middlelayer 114 (and again at the bottom layer 112) is generally described asfollows. Considering the “G” block in the top layer 116, the motionvector G may be copied to the middle layer 114 as motion vector searchcenters in the current group of blocks corresponding to the “G” block(e.g., G1=G, G2=G, G3=G and G4=G). A first pass refinement of the middlelayer motion vectors A1-L4 may then be conducted by the circuit 102 inthe step 140 to improve the accuracy of the middle layer motion vectorsA1-L4 (e.g., usually G1≠G, G2≠G, G3≠G and G4≠G). The refinement motionestimations generally use the top layer motion vectors A-L as seeds ofsearch centers for the motion estimation operations. The overallrefinement may be conducted in a left-to-right pattern in each row androw-by-row from top to bottom of the picture. Other patterns may beimplemented to meet the criteria of a particular application.

One or more second pass motion estimation refinements may be conductedby the circuit 102 in the step 142 for each of the middle pictureblocks. The second pass refinements may begin either (i) during thefirst pass or (ii) after the first pass has completed. A second passrefinement of a particular block generally uses one or more the refinedmotion vectors from one or more outer neighbor blocks as new searchcenter seeds. For example, the “G1” block may have a motion estimationperformed using the motion vector F2 (previously refined in the firstpass) as the search center seed. An optional motion estimation may alsobe performed using the motion vector C3 (previously refined in the firstpass) as the search center seed. The single motion vector among thethree motion vectors with the minimum motion estimation score may becarried forward and stored in the circuit 104 as the refined motionvector for the “G1” block.

In some embodiments, a neighboring motion vector may be used to replacethe current motion vector only if the corresponding neighboring motionestimation score is less than a ratio of the current motion estimationscore. The ratio is based on an observation that if an object edge islocated in the current block, the neighboring motion vector may belocked properly on the object. Therefore, the neighboring motion vectormay be a better candidate to track the object. Every non-top layer mayperform the above motion estimation refinement to improve the quality ofthe motion vectors used to seed the following layer.

Referring to FIG. 5, a flow diagram of a second example implementationof a motion estimation refinement method 132 b is shown. The method (orprocess) 132 b may be implemented by the apparatus 102 and used as thestep 132 of the method 120. The method 132 b generally comprises a step(or block) 150, a step (or block) 152, a step (or block) 154 and a step(or block) 156.

In the step 150, the circuit 102 may conduct a motion estimationrefinement of a particular block (e.g., the “G2” block) using thepulled-down motion vector (e.g., the motion vector G) from the toppicture as an initial search center seed. One or more additional motionestimation refinements for the particular block may then be performed inthe step 152 using one or more neighboring motion vectors (e.g., themotion vector C4 and the motion vector H1), if available, as additionalsearch centers. The circuit 102 may identify a best motion vectorgenerated by the motion estimation refinements as the refined motionvector for the particular block in the step 154. If more blocks areavailable to refine (e.g., the YES branch of step 156) the circuit 102may move to the next block in the step 158. If no more blocks areavailable to refine (e.g., the NO branch of step 156), the refinement atthe current level may end.

Regarding the motion vectors C4 and H1, a left-to-right, top-to-bottomscan through the middle picture may result in the motion vector C4 beingpreviously refined (e.g., C4≠C) before being used as a seed for the “G2”block. However, the motion vector H1 may be unrefined (e.g., H1=H) whenused as the search center seed for the “G2” block. In some embodiments,all of the neighboring motion vectors may be the unrefined motionvectors pulled down from the higher layer (e.g., C4=C and H1=H).

In some embodiments, the refined motion vectors of the horizontalneighbors may be used as the search center seeds for some motionestimation refinements while the unrefined (e.g., pulled-down from theabove layer) motion vectors of the vertical neighbors may be used as theseeds for other motion estimation refinements. The above mixed methodmay reduce a delay between searches and/or passes in someimplementations, while giving a partial amount of the coding gain byusing the refined current layer neighboring motion vectors whereverpossible.

The motion estimation refinement searches may be conducted using fewer(e.g., only 1) neighboring motion vectors (e.g., the adjoininghorizontal neighbor) or additional (e.g., 3, 4 or 5) neighboring motionvectors (e.g., the adjoining horizontal neighbor, the adjoining verticalneighbor, the diagonally adjoining neighbor and/or one or more adjacentneighbors that would normally be a neighbor of other blocks in thecurrent group of blocks). For example, the refinement of the motionvector G1 may include all of the motion vectors F2, B4, C2, D3 and K2.Other combinations of neighboring motion vectors may be used to meet thecriteria of a particular application.

Referring to FIG. 6, a flow diagram of a third example implementation ofa motion estimation refinement method 132 c is shown. The method (orprocess) 132 c may be implemented by the apparatus 102 and used as thestep 132 of the method 120. The method 132 c generally comprises a step(or block) 160, a step (or block) 162, a step (or block) 164, a step (orblock) 166, a step (or block) 168 and a step (or block) 170.

Choosing a subset of the neighbors to search may be desirable. Aselection of appropriate neighbors may be based on a spatial distancebetween the motion vectors. For example, a subset of the one or moreneighboring motion vectors that are the furthest (e.g., in Euclidean orL1-norm distance) from the current block motion vector search center maybe identified for the one or more additional searches.

In the step 160, the circuit 102 may refine the motion vector of aparticular block using the pulled-down motion vector as the searchcenter seed. The “furthest” one or more neighboring motion vectors maybe identified by the circuit 102 in the step 162. Additional refinementsmay be conducted by the circuit 102 in the step 164 using the identifiedmotion vectors as the seeds. Thereafter, a best motion vector may bedetermined in the step 166 and stored in the circuit 104. If more blocksare available (e.g., the YES branch of step 168), the circuit 102 maymove to the next block in the step 170. Otherwise (e.g., the NO branchof step 168), the refinement at the current layer may end.

A selection of the neighboring motion vectors may also be made to createa largest spatial spread among the search centers (e.g., the set thatencompasses the largest region/area). In the method 132 c, the step 162may identify one or more neighboring motion vectors producing awidest-spread search area. After that, the method 132 c may continue asbefore, refining the motion estimations of the particular block in thestep 164 and determining the best motion vector in the step 166.

Referring to FIG. 7, a diagram of an example picture 172 divided intoblocks and sub-blocks is shown. The example is based on the partitionsof the H.264 recommendation. Other size partitions may be implemented tomeet the criteria of a particular application.

A current picture undergoing a motion estimation may be initiallydivided into multiple blocks (e.g., 8×8 pixel blocks). Each of theblocks may have an associated motion vector, labeled A-L in the figure.A subsequent partitioning may divide the current picture into multiplesub-blocks (e.g., 8V×4H pixel blocks). Each of the sub-blocks may haveone or more associated motion vectors, labeled A1-L2 in the figure.

Referring to FIG. 8, a flow diagram of an example implementation of amethod 180 of a sub-block motion estimation is shown. The method (orprocess) 180 may be implemented by the apparatus 100. The method 180generally comprises a step (or block) 182, a step (or block) 184, a step(or block) 186, a step (or block) 188, a step (or block) 190, a step (orblock) 192 and a step (or block) 194.

The present invention may be used in a non-hierarchical motionestimation. In particular, modern coding standards generally permitusing multiple different block sizes for motion compensation. In amotion estimation operation that attempts to reduce the complexity ofsearching over the multiple different block sizes, search results fromlarger block sizes may seed the search centers for smaller block sizes.Implementation of the present invention may increase a quality of theencoding where limited search resources are available. The method 180may also be applied even if the motion search is not hierarchical. Forexample, the method 180 may be initially applied at a macroblock level(e.g., 16×16 pixels), then down to a block level (e.g., 8×8 pixels) andfinally a sub-block level (e.g., 8×4, 4×8 and 4×4 pixels).

In the step 182, the circuit 102 may divide the current picture intoblocks. A motion estimation may then be conducted for each of themacroblocks by the circuit 102 in the step 184 to estimate theassociated motion vectors (e.g., motion vectors A-L). The currentpicture may be further divided by the circuit 102 into sub-blocks in thestep 186. The motion vectors A-L from the block level (or layer) may becopied into the corresponding motion vectors A1-L2 by the circuit 102 inthe step 188. Each block level motion vector may be copied into multiple(e.g., 2) of the sub-block motion vectors (e.g., A1=A and A2=A).

A motion estimation refinement may be processed by the circuit 102 inthe step 190 to adjust the sub-block level motion vectors A1-L2 (e.g.,usually A1≠A and A2≠A). The refinement step 190 may use any of theseveral refinement techniques described above (e.g., methods 132 a-132c). If smaller sub-blocks (e.g., 4×4 pixels) are available to refine(e.g., the YES branch of step 192), the steps 186-190 may be repeatedusing the motion vectors from the current sub-blocks as seeds to thesmaller sub-blocks. Once all of the smallest sub-blocks have beenadjusted (e.g., the NO branch of step 192), the circuit 102 may storethe best motion vectors in the circuit 104 per the step 194.

The present invention generally uses multiple (e.g., 2) “passes” at eachlayer or level. An advantage of the two pass technique is that refined(e.g., first pass) motion vectors (searched and refined around searchcenters received from a higher layer of the motion estimation hierarchy)from all neighboring blocks may be available for the second passrefinement stage (which uses the refined neighboring motion vectors forsearch centers). However, non-causal neighbors (e.g., if processingproceeds down and to the right, the left neighbors and the aboveneighbors are causal neighbors while the below neighbors and the rightneighbors are non-causal neighbors) may be available for the refinement.For example, a single pass of the motion estimation refinementprocessing (due to limited resources) may be achieved by utilizing themotion vectors from casual neighbors and the motion vectors from thenon-casual neighbors.

The functions performed by the diagrams of FIGS. 1-8 may be implementedusing a conventional general purpose digital computer programmedaccording to the teachings of the present specification, as will beapparent to those skilled in the relevant art(s). Appropriate softwarecoding can readily be prepared by skilled programmers based on theteachings of the present disclosure, as will also be apparent to thoseskilled in the relevant art(s).

The present invention may also be implemented by the preparation ofASICs, FPGAs, or by interconnecting an appropriate network ofconventional component circuits, as is described herein, modificationsof which will be readily apparent to those skilled in the art(s).

The present invention thus may also include a computer product which maybe a storage medium including instructions which can be used to programa computer to perform a process in accordance with the presentinvention. The storage medium can include, but is not limited to, anytype of disk including floppy disk, optical disk, CD-ROM,magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, Flash memory,magnetic or optical cards, or any type of media suitable for storingelectronic instructions.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the scope of the invention.

The invention claimed is:
 1. A method of motion estimation refinement,comprising the steps of: (A) generating an initial motion vector byconducting a first motion estimation on an initial block in a picture,said initial block covering an initial area of said picture; (B)generating a current motion vector by conducting a second motionestimation on a current block in said picture, (i) said current blockcovering a subset of said initial area and (ii) said second motionestimation being seeded by said initial motion vector; (C) generating atleast one additional motion vector by conducting at least one thirdmotion estimation on said current block, said at least one third motionestimation being seeded respectively by at least one neighboring motionvector spatially adjacent to said current motion vector; and (D)generating a refined motion vector of said current block by storing in amemory a best among said current motion vector and said additionalmotion vector.
 2. The method according to claim 1, wherein (i) saidfirst motion estimation is conducted at a top layer of a plurality oflayers in a hierarchical motion estimation and (ii) both said secondmotion estimation and said third motion estimation are conducted at alower of said layers below said top layer.
 3. The method according toclaim 1, wherein (i) said current block comprises a sub-block of saidinitial block and (ii) all of said first motion estimation, said secondmotion estimation and said third motion estimation are conducted at asame resolution of said picture.
 4. The method according to claim 1,wherein (i) said neighboring motion vector comprises a first motionvector of a first block spatially adjoining said current block and (ii)said first motion vector is not copied from said initial motion vector.5. The method according to claim 4, wherein (i) said neighboring motionvector further comprises a second motion vector of a second blockspatially adjoining said current block, (ii) said second block isdifferent than said first block and (iii) said second motion vector isnot copied from said initial motion vector.
 6. The method according toclaim 5, wherein (i) said neighboring motion vector further comprises athird motion vector of a third block spatially diagonal to said currentblock, (ii) said third block is different than both said first block andsaid second block and (iii) said third motion vector is not copied fromsaid initial motion vector.
 7. The method according to claim 1, furthercomprising the step of: identifying said neighboring motion vector as aparticular motion vector among a plurality of candidate motion vectorsof a plurality of candidate blocks spatially adjacent to said currentblock such that said particular motion vector is spatially furthest fromsaid current motion vector.
 8. The method according to claim 1, furthercomprising the step of: identifying said neighboring motion vector as atleast two particular motion vectors among a plurality of candidatemotion vectors of a plurality of candidate blocks adjacent to saidcurrent block such that said particular motion vectors cover a largestsearch area among said candidate motion vectors.
 9. The method accordingto claim 1, further comprising the step of: motion refining saidneighboring motion vector before seeding said third motion estimation.10. The method according to claim 1, wherein said neighboring motionvector is not motion refined before seeding said third motionestimation.
 11. An apparatus comprising: a memory; and a circuitconfigured to (i) generate an initial motion vector by conducting afirst motion estimation on an initial block in a picture, said initialblock covering an initial area of said picture, (ii) generate a currentmotion vector by conducting a second motion estimation on a currentblock in said picture, (a) said current block covering a subset of saidinitial area and (b) said second motion estimation being seeded by saidinitial motion vector, (iii) generate at least one additional motionvector by conducting at least one third motion estimation on saidcurrent block, said at least one third motion estimation being seededrespectively by at least one neighboring motion vector spatiallyadjacent to said current motion vector and (iv) generate a refinedmotion vector of said current block by storing in said memory a bestamong said current motion vector and said additional motion vector. 12.The apparatus according to claim 11, wherein (i) said first motionestimation is conducted at a top layer of a plurality of layers in ahierarchical motion estimation and (ii) both said second motionestimation and said third motion estimation are conducted at a lower ofsaid layers below said top layer.
 13. The apparatus according to claim11, wherein (i) said current block comprises a sub-block of said initialblock and (ii) all of said first motion estimation, said second motionestimation and said third motion estimation are conducted at a sameresolution of said picture.
 14. The apparatus according to claim 11,wherein (i) said neighboring motion vector comprises a first motionvector of a first block spatially adjoining said current block and (ii)said first motion vector is not copied from said initial motion vector.15. The apparatus according to claim 14, wherein (i) said neighboringmotion vector further comprises a second motion vector of a second blockspatially adjoining said current block, (ii) said second block isdifferent than said first block and (iii) said second motion vector isnot copied from said initial motion vector.
 16. The apparatus accordingto claim 15, wherein (i) said neighboring motion vector furthercomprises a third motion vector of a third block spatially diagonal tosaid current block, (ii) said third block is different than both saidfirst block and said second block and (iii) said third motion vector isnot copied from said initial motion vector.
 17. The apparatus accordingto claim 11, wherein said circuit is further configured to identify saidneighboring motion vector as a particular motion vector among aplurality of candidate motion vectors of a plurality of candidate blocksspatially adjacent to said current block such that said particularmotion vector is spatially furthest from said current motion vector. 18.The apparatus according to claim 11, wherein said circuit is furtherconfigured to identify said neighboring motion vector as at least twoparticular motion vectors among a plurality of candidate motion vectorsof a plurality of candidate blocks adjacent to said current block suchthat said particular motion vectors cover a largest search area amongsaid candidate motion vectors.
 19. The apparatus according to claim 11,wherein said circuit is further configured to motion refine saidneighboring motion vector before seeding said third motion estimation.20. An apparatus comprising: means for storing; and means for (i)generating an initial motion vector by conducting a first motionestimation on an initial block in a picture, said initial block coveringan initial area of said picture, (ii) generating a current motion vectorby conducting a second motion estimation on a current block in saidpicture, (a) said current block covering a subset of said initial areaand (b) said second motion estimation being seeded by said initialmotion vector, (iii) generating at least one additional motion vector byconducting at least one third motion estimation on said current block,said at least one third motion estimation being seeded respectively byat least one neighboring motion vector spatially adjacent to saidcurrent motion vector and (iv) generating a refined motion vector ofsaid current block by storing in said means for storing a best amongsaid current motion vector and said additional motion vector.